1. Field of the Invention
The field of the invention comprises apparatus and methods for manufacturing semiconductor devices.
2. Discussion of the Related Art
For the past 20 years, the integrated circuit (IC) device density has doubled about every 18 months. As device density increases, the space between semiconductor device features must therefore decrease. However, as the space between device features decreases, the ability to electrically isolate the devices becomes more difficult. To provide the electrical isolation as well as mechanical structure to fabricate the devices, the narrow spaces are filled with silicon dioxide or other suitable insulator. To separate the devices within the silicon substrate, spaces are provided by means of shallow trenches. These trenches are filled with silicon dioxide which can be derived from the reaction of tetraethylorthosilicate (TEOS) and ozone.
The fabrication of shallow trenches is currently carried out by providing a semiconductor substrate, typically silicon wafer with a thermal oxide surface, which is manufactured by exposing a surface of silicon to an oxidizing agent such as oxygen at high temperatures to create a surface of SiO.sub.2.
Similarly, polysilicon interconnect layers as well as conductive metal interconnects have spaces which are to be filled with silicon dioxide or other suitable dielectric material to insulate the conductive layers from each other. The dielectric material, such as SiO.sub.2, can be made of un-doped silicate glass (USG) or non-doped silicon glass (NSG). These terms are considered to be equivalent in this application. USG is deposited using the precursors, TEOS and ozone, using a chemical vapor deposition (CVD) processing.
Conventionally deposited films of USG based on TEOS/ozone technology, however, have several drawbacks. Of these, one of the most important is the sensitivity of the deposited film to the condition of the substrate on which the film is deposited. This is termed "surface sensitivity." {See Kwok et al., J. Electrochem. Soc., 141(8):2172-2177 (1994); Matsuura et al., Proceedings of the 22.sup.d International Conference on Solid State Devices and Materials, Sendai, pp:239-242 (1990); Fujino et al., J. Electrochem. Soc. 138(2):550-554 (1991); and Fujino et al., J. Electrochem. Soc. 139(6):1690-1692 (1992), each incorporated herein fully by reference.} Surface sensitivity is characterized by inconsistent and variable deposition rates and increased roughness of the resulting films as the process conditions are varied. The process conditions of interest are deposition temperature, deposition pressure, mole-fraction of the reactants (e.g., TEOS and ozone), and possibly some hardware conditions specific to the design of the reactor used to deposit these films.
Increasing the ozone concentration can result affect the deposition rate and increased surface roughness, as reflected both in direct measurements of surface features using scanning electron microscopy, as well as increased wet etching rates observed with aqueous HF solutions. One possible hypothesis to account for surface roughness is the nucleation mechanism, also known as an "island growth mechanism" originating on the surfaces being covered. Depending on the molecular makeup of the surface and its state, the TEOS ozone begins deposition in a granular form. As the film becomes thicker, this granular structure can be seen as surface roughness. This is in contrast with a smooth, "layered growth mechanism" which results in an even, smooth layer of deposited material.
Surface roughness introduces a number of adverse effects for processing the TEOS ozone films and films subsequently deposited on the TEOS ozone films. As photoresist is coated on the wafer surface to form holes (vias) for interconnecting conductive layers, the photoresist's surface adhesion is decreased on rough surfaces. With the exposure of the photoresist to the stepper light radiation, the granular surface introduces many reflections resulting in poor dimensional definition of the photoresist layer. During etching, the granular surface reduces adhesion with the possibility of delamination of the photoresist layer.
Additionally, as the gaps between semiconductor device features decreases, these gaps becomes increasingly more difficult to fill adequately. As the surface films increase in thickness, the corresponding film does not completely fill the gap, resulting in the formation of a "void." This is especially the case if there is surface sensitivity of deposition of materials within the gap. Films deposited at low ozone concentration exhibit conformal coating of device features, resulting in the formation of a void as the gap becomes filled.
These unfilled gaps, or "voids" can trap contaminants which can degrade the integrated circuit device, and are not effective dielectrics. The presence of voids therefore decreases device reliability of the device. However, TEOS/ozone films deposited using high ozone concentrations are known to exhibit better gap filling properties and have better in situ flow characteristics than low ozone films {Kwok et al., J. Electrochem. Soc. 141(8):2172-2177 (1994)}.
Surface sensitivity is observed for several types of substrates, including silicon dioxide formed either as thermal oxide, or as SiO.sub.2 deposited through TEOS ozone or TEOS oxygen plasma processes (PECVD). The SiO.sub.2 can be either phosphorous doped silicate glass (PSG), spin on glass (SOG), borophosphorous silicate glass (BSG), or combinations of PSG and BSG. Additionally, surface sensitivity can be a problem for dielectric layers deposited on silicon nitride (Si.sub.3 N.sub.4).
Thus, the semiconductor industry is interested in decreasing the effects of surface sensitivity and ensuring good gap filling with high quality dielectric materials. However, to date the problems of surface sensitivity and poor gap filling have been not been adequately addressed.
I Surface Sensitivity
Several approaches have been used to treat semiconductor surfaces. Maeda et al. U.S. Pat. No. 5,484,749 described a process whereby semiconductor devices were exposed to heat and high frequency plasma to treat the surface of an SiO.sub.2 layer prior to deposition of a TEOS ozone film. However, this process requires additional costly plasma treatment equipment.
Another approach by Maeda et al., U.S. Pat. No. 5,051,380 was to deposit the USG film discontinuously. A first layer of USG is deposited using a low ozone concentration, and then a subsequent USG film is deposited using a higher ozone concentration. However, USG films using low ozone concentration have less desirable film qualities, such as high water content and absorption, high etch rates, and shrinkage during high temperature annealing, which induces stress and formation of voids or seams. {Kwok et al., J. Electrochem. Soc. 141(8):2172-2177 (1994).} Furthermore, the thickness needed to overcome surface sensitivity of a low ozone USG film may be as great as 1000 .ANG., thus filling the gap with poor quality oxide and preventing filling the gap with high quality, high-ozone oxide. As gaps decrease to 0.25 .mu.m, after a first layer of a low ozone USG film, only 500 .ANG. would be available for a second layer of a effective high ozone USG material to fill the gaps.
An additional approach is to modify the surface of the USG film. Maeda et al., U.S. Pat. No. 5,387,546 taught the exposure of deposited semiconductor films to ultraviolet radiation during heating. The ultraviolet radiation was produced by a mercury lamp which generates electromagnetic radiation with wavelengths of 185 nanometers (nm) and 254 nm, as well as some longer wavelength radiation. However, because this process is carried out on layers of USG film which have already been deposited, it does not deal with the problem of surface sensitivity. Thus, there is a need for improved ways of reducing surface sensitivity.
II Gap Filling
Attempts to improve the filling of gaps include the use of nitrogen gas (N.sub.2) plasma to selectively treat dielectric materials deposited over conductive lines to decrease the rate of deposition of USG films at those locations. Jang et al., U.S. Pat. No. 5,536,681. By decreasing the rate of deposition over the conductive lines, relatively more dielectric material can be deposited within the gaps, resulting in decreased formation of voids. However, this process requires at least 3 additional steps and additional equipment, is expensive, and requires additional processing time, and therefore is not efficient.
Chen, U.S. Pat. No. 5,489,553, exposed a thermal oxide layer to HF. HF is a strong acid which removes all surface silicon dioxide, exposing bare silicon for subsequent deposition of TEOS ozone silicon dioxide films. The formation of SiOF moieties on the surface resulted in improved conformal deposition of oxide; however, the introduction of fluorine atoms into the film can result in the liberation of fluoride ions which can introduce reliability problems as the fluorine ions combine with moisture to form HF. Moreover, the presence of fluorine atoms may themselves result in surface sensitivity. { See Kwok et al., J. Electrochem. Soc. 141:2172, incorporated herein fully by reference.}
Therefore, there is a need for improved gap filling processes which can be carried out without the introduction of harmful species into the dielectric films.